High Intensity Discharge (HID) lamps are high-efficiency lamps used to generate high levels of lighting from a relatively small source, especially for industrial and infrastructure lighting applications. The term “HID lamp” may include mercury vapor lamps, metal halide lamps, and high pressure sodium lamps. Metal halide lamps, for instance, are commonly used in large spaces that require a high level of brightness at relatively low cost.
Typical construction of a HID lamp comprises a pair of electrodes, constructed of a refractory metal such as tungsten, enclosed within an arc bulb containing a pressurized gas. In steady operation of the HID lamp, light is generated by the hot gas creating a plasma discharge when an electrical current is conducted through the gas, between the electrodes.
To initiate the arc between the electrodes, the gas must first be ionized. This is typically done at the lamp ignition stage, such as via a high voltage spike, up to and even exceeding 5 kV in magnitude, between the electrodes. The ignition state being reached is characterized by a transitory phase of intense luminous output and heat generated from the plasma discharge by passage of electric current through the pressurized gas between the electrodes.
HID lamps typically require an electrical ballast for providing electrical power for the operation of the lamp, including both ignition state and subsequent steady state operation. Due to the varying voltage requirements associated with the progressive stages in operating the HID lamp, the electrical ballast circuitry needs to tailor the voltage protocols accordingly. For instance, just prior to ignition, when the electrodes are cold, a sufficiently strong voltage must be applied to generate thermionic emission, where electrons are lifted off the surface of the electrodes. Electrical ballast circuitry regulates the flow of current to facilitate ignition and subsequent steady state operation of the lamp. Circuitry components of the ballast, in addition to inductive and resistive components, may include a transformer with an ignition component to drive the lamp to an ignited state. Once the lamp transition out of the ignition state, the ballast then reduces the voltage applied to the lamp while increasing the lamp current. Thereafter, the current is regulated for the lamp to operate in steady state.
Certain drawbacks or adverse consequences are associated with achieving lamp ignition via the sudden, almost discontinuous nature of the high voltage spike. Sputtering at the electrodes produces particulate removal and scattering of the electrode surface material, changing the geometry of the electrode tips, and degrading the electrode characteristics cumulatively each time a high voltage ignition spike is applied to the lamp. With time, that particulate material condenses on, and darkens, the inner surface of the lamp. Since the electrodes play a significant role in initial striking of the electrical arc and in determining the luminosity of the lamp, light transmission efficiency and lamp performance degrades as the lamp darkens. The temperature of the lamp tube may be higher in steady state operation, also, effectively decreasing the useful life of the lamp. It is also commonplace practice for lamp installers to over-specify the lamp wattage for a given the application, typically by around 30%, in anticipation of lamp darkening and lessened lighting efficiency over the life of the lamp; obviously a wasteful, though rational, practice.